In known apparatus of this kind the powder is shaken by a vibrator in a powder container constructed in the form of a cylinder, prior to deposition on the metering plate. From the metering plate, the powder is conveyed via a doctor member, which scrapes the powder off the plate, into a cone with a powder supply tube connected thereto, to the burner. The powder conveying medium is likewise gas. The unit is always under a relatively high pressure of approximately 3 bar.
Owing to the strong vibrations required, unmixing effects of the grain fraction are unavoidable, and due to accumulations of the powder, irregular quantities are thrown into the cone by the doctor member.
Another known apparatus for the metered supply of powder to a powder processing unit comprises a worm or a turbine wheel for metering. In this case the risk exists that the cavities of the worm or the cavities of the turbine wheel are not always filled perfectly, and high wear occurs. The filling conditions are therefore always changing, so that accurate reproducible metering is possible only with difficulty.
Finally, a disc having a number of holes at the periphery has become known for metering. The holes are filled with powder, and a gas jet then presses the powder out of the holes again. With this apparatus, too, perfectly reproducible metering and a uniform supply of powder to the powder processing unit cannot be obtained.